docs: def all_arcs(constraints):
      """
      For each constraint ((X, Y), const) adds:
          ((X, Y), const)
          ((Y, X), const)
      """
      arcs = set()
  
      for neighbors, constraint in constraints:
          if len(neighbors) == 2:
              x, y = neighbors
              list(map(arcs.add, ((x, y), (y, x))))
  
      return arcs